1. Field of the Invention
The current invention relates to clothing for paper machines and relates in particular to non-woven clothing and to the manufacture of same.
2. Description of the Related Art
Paper machines are utilized for the production of fibrous webs, for example different types of papers, cartons, cardboards and similar nonwovens. In this document the term “paper” is representative for these types of fibrous webs.
The production of a fibrous web starts in the forming section of a paper machine with the deposit of a fibrous stock suspension on clothing, or respectively with the introduction of a fibrous stock suspension into the gap which is formed between two clothings. As a rule, clothing is in the embodiment of endless belts which, rerouted over rollers, rotate within a certain section of the paper machine. The paper-side surface of the clothing carries the fibrous suspension, or respectively the fibrous web or fibrous nonwoven web resulting from dewatering. The surface of the clothing running over the rolls is referred to below as the running-side surface. The clothing is equipped with passages through which water is drawn from the paper-side surface to the running surface.
Clothing currently used in the forming section of paper machines as forming fabric consists of woven material. Woven clothing features uniform structures with a repeat basic pattern. The forming fabrics are generally composed of several woven layers having different thread sizes and thread directions. Because of their different weave structures, the individual layers of such clothing not only have water permeability differing from each other but, since the openings or passages in the paper-side layers are regularly covered by threads of woven layers arranged beneath them also lead to laterally local variations in permeability of the forming fabric. Since a laterally varying permeability results in locally varying dewatering velocity of the fibrous web, visible markings in the fibrous web or paper web with a uniform arrangement following the weave pattern are the result. Since lesser dewatered regions in a web also have a lower fiber density, lateral permeability fluctuations moreover compromise the paper quality also through this effect.
Woven types of clothing have a lesser flexural strength and therefore are often prone to crease formation during rotation through the machine. The use of monofilaments of various materials, for example a combination of yarns consisting of polyethylene terephthalate (PET) and polyamide (PA) on the running side of a clothing leads to protruding or curling of forming fabric edges, due to the different characteristics of these materials in regard to water absorption, expansion, etc.
Since clothing cannot be woven as an endless belt, both ends of a continuously long woven belt must be joined with each other in order to form an endless belt. In order to avoid irregularities at the joint location which would lead to marking of the web, the connection is made through a complicated woven seam structure, whereby the ends of warp and weft threads allocated to each other are spliced together at the connection location of the woven belt, offset according to a certain pattern. This joining technique is very complex and is reflected in accordingly high production costs for woven endless clothings.
As an alternative to woven clothing, types of clothing were suggested which are produced from nonwoven material webs. In international patent specification CA 1 230 511 and U.S. Pat. No. 4,541,895 an example of a clothing is cited which is formed from a laminate of several layers of nonwoven, water-impermeable materials into which openings are introduced for the purpose of dewatering. Joining of the individual layers of the laminate occurs, for example through ultrasonic welding, high frequency welding or thermal welding. The dewatering holes are introduced into the laminate preferably by means of laser drilling. The welded seam of one layer can be arranged offset to that of the other layers, whereby the welded seams moreover can be arranged at an angle to the direction of travel of the endless belt in order to avoid visible thickening of the clothing. However, to produce such film laminates in the dimensions necessary for forming fabrics is very expensive. Such multilayer film laminates are moreover very stiff and have a tendency to delaminate under the conditions prevailing during use in the forming section of a paper machine.
If polymer belts are used to produce clothing for paper machines, then these must be drawn in the direction of travel of the clothing. Otherwise the clothing is irreversibly stretched under the tensile stresses prevailing during operation and would therefore become unusable in a very short time. In industrial scale applications of paper machines, clothing having widths of approximately 8 to 12½ meters (m) are typically used. Non-directionally drawn polymer belts are, however, currently only available in widths of typically approximately 1 to approximately 2 meters. Biaxially drawn belts are currently offered at approximately 4 m wide. Therefore, to produce clothing, several laterally adjacent polymer belts must be joined together. In order to produce clothing in the embodiment of an endless belt the ends of the belt must moreover be joined. At the location of the joint the mechanical stability is diminished compared to the full material.
To solve the problem, U.S. Patent Application Publication No. 2010/0230064 suggests clothing for use in paper machines which is produced from a spirally wound polymer ribbon. The width of the polymer ribbon is considerably narrower than the width of the clothing produced therefrom, whereby the longitudinal direction of the polymer ribbon—except for the slanting provided by the winding pitch—is consistent with the direction of travel of the clothing. The side edges located opposite each other of adjoining winding cycles of the polymer ribbon are welded together to form a closed running surface. Since the welded seam is arranged in a relatively small angle to the direction of travel of the clothing, the tensile stress components acting transversely to the welded seam are small, so that in an ideal situation the material in the region of the welded seam is not unduly stressed. The production of clothing from a spirally laid polymer ribbon is however very expensive, since it requires a special welding device, whereby either the welding apparatus has to be guided at high precision several times along the welding line around the clothing or whereby the clothing must be moved with the rotating welding line relative to the welding apparatus. Moreover, the edges of the clothing must be trimmed after the welding process in order to obtain clothing having a uniform width. Consequently, the welded seam encounters one of the side edges of the clothing at a pointed angle, thus providing a weak point for tearing of the clothing, due to the structurally weaker welded seam, compared to the polymer ribbon.
What is needed in the art is a clothing for paper machines which is film-like, has high mechanical stability and tensile strength, is sufficiently wide for use in industrially employed paper machines and which can be manufactured with conventional means.